- Assemble x-anthropic-billing-header strictly following official 2.1.258 binary sequence: cc_version -> cc_entrypoint -> cch -> cc_workload -> cc_is_subagent -> cc_prev_req -> cc_prompt_id.
- Track upstream request-id in thread-safe bounded session continuity state and inject into subsequent turns as cc_prev_req.
- Generate and enforce RFC 4122 UUIDv4 cc_prompt_id per user prompt turn, preserving it across tool continuations while suppressing it on probes and title helpers.
- Detect subagents via agent headers and session metadata to emit cc_is_subagent=true.
- Harmonize diagnostics and continuity lifecycles to prevent uncommitted or truncated streams from corrupting state.
- Exclude x-anthropic-billing-header system blocks from sensitive-word zero-width space injection.
- Introduced agent-specific cache key isolation to ensure distinct prompt caching across agents.
- Improved reasoning replay handling for Claude models, including cumulative tool turn restoration and validation.
- Enhanced HTTP and WebSocket session consistency during execution scope and cache replay synchronization.
- Added new unit tests and extended coverage for prompt caching, agent isolation, and replay cache functionality.
Closes: #4352
- Introduced `ClaudeCodeSessionID` resolution logic, preferring headers over payload metadata.
- Added `ClaudeCodePromptCache` to map sessions to stable prompt cache keys.
- Refactored existing logic to integrate `ClaudeCodePromptCache` for session-based handling.
- Included extensive unit tests to validate session ID extraction, cache reuse, and header prioritization.
